Kinetic studies of dexamethasone degradation in aqueous solution via a photocatalytic UV/H2O2/MgO process

Wastewaters discharged from different industries and hospitals may contain pharmaceuticals, especially dexamethasone (DEX). Thus, we applied the UV/H2O2 photocatalytic method in the presence of the MgO nanoparticles to remove dexamethasone from synthetic wastewater. Moreover, the effects of parameters such as pH (3–11), hydrogen peroxide concentration (1–8 mM), initial DEX concentration (5–30 mg/L), and catalyst dosage (0.01–0.2 g/L) during the reaction times (0–30 min) were investigated. Furthermore, the efficiency of UV/H2O2 in the presence and absence of catalysts was investigated. The photocatalyst is characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and Fourier-transform infrared spectroscopy (FTIR) techniques. It was found that the removal rate was enhanced by decreasing pH and the initial dexamethasone concentration. The removal rate was enhanced somewhat with concentrations of hydrogen peroxide and MgO. In the case of UV/H2O2/MgO, 87% removal efficiency was achieved, under the optimal conditions: pH 3, contact time of 30 min, dexamethasone concentration of 20 mg/L, H2O2 of 0.5 mM, and UV radiation of 55 watts. The kinetic data indicated that the reaction followed the second-order kinetic model. The results showed that the UV/H2O2 photochemical process can efficiently remove dexamethasone from aqueous in the presence of a MgO catalyst, and the mineralization efficiency was reached at about 98%.

www.nature.com/scientificreports/ the possibility of producing cost-effective and efficient photocatalysts 11,12 . In a photocatalytic reaction, a catalyst is exposed to visible or UV light irradiation to generate hydroxyl radicals 13 . In photocatalytic processes, semiconductors like TiO 2 , ZnO, CdS and ZrO 2 have mainly been used for organic matter degradation 14 . In general, nanoparticles are suitable for chemical reactions and the adsorption of different organic materials because of their high specific surface area 15 . Among these nanoparticles, MgO, which is a basic oxide, has various applications as a catalyst. Magnesium oxide (MgO) is a semiconductor whose unique chemical, mechanical, optical, and electrical properties, wide energy band gap, stability, inexpensive, and non-toxicity have made it very attractive for application photocatalytic processes 16 . It has been observed that the use of MgO nanoparticles in concert with catalytic ozonation can enhance the degradation of organic pollutants 17 . In this research, we tried to investigate the catalytic impact of MgO nanoparticles alongside UV radiation for hydrogen peroxide activation. Thus, in the presence of the MgO nanoparticles, the performance of the UV/H 2 O 2 process in DEX removal from the aqueous environment was assessed.

Materials and methods
Chemicals and photoreactor. All the chemicals utilized in this study, H 2 O 2 , NaOH and H 2 SO 4 , and radical scavengers: ascorbic acid (AA), ethylenediamine tetraacetic acid (EDTA), and tert-butyl alcohol (TBA), were purchased from Sigma Aldrich and Merck Co., Germany. The MgO nanoparticle powder was purchased from ASPI Co. and sodium dexamethasone phosphate (C 22 H 28 FNa 2 O 8 P) was bought from Darou Pakhsh Pharmaceutical Co. For the tests, a 2-L stainless steel photoreactor, which was equipped with quartz glass and a 55-W low-pressure lamp (Philips Co.), was employed. A pump was used in order to continuously mix the samples. Figure 1 presents all the details of the reactor. To determine the dexamethasone degradation intermediates, liquid chromatography-mass spectrometry (LC-MS; Shimadzu LC/MS 2010 A) was used 18,19 .
Experimental procedures. All experiments were performed in a batch-flow pilot. In the current research, at the fixed intensity of ultraviolet light, the impacts of a pH of 3-11, an initial DEX content of 5-30 mg/L, a hydrogen peroxide content of 1-8 mM, a MgO dose of 0.01-0.2 g/L, and a contact time of 0-30 min on the process performance were studied. All runs were carried out in triplicate at room temperature (25 °C), and average figures were recorded. Both first-and second-order linear kinetics were used to obtain the best-fitting model for the removal process. In this work, a solution containing 100 mg/L of the pollutant was prepared. Next, the working concentrations were prepared from this stock solution. A UV/Vis spectrophotometer (wavelength = 241 nm) (DR 5000, Hach Co., Germany) was employed to determine the contents of the DEX solution. Furthermore, as Table1. The characteristics of dexamethasone. UV/MgO processes. ), hole (h + ), and hydroxyl radical ( · OH), AA (0.2 mol/L), EDTA (0.2 mol/L), and TBA (0.2 mol/L) were used, respectively. In order to determine the mineralization rate of the pollutant during the process, the total organic carbon (TOC) of the experiments was detected using a Shimadzu 5000 TOC analyzer. Moreover, the chemical oxygen demand (COD) was determined according to the procedure expressed in Standard Methods (Federation and Association). A digital pH meter (Hach) was applied to determine the pH values. In the end, all the charts were plotted and the data was analyzed by means of Excel 2013. X-ray powder diffraction (XRD) was used to determine the crystal phases of the MgO nanoparticles (Rigaku Ultima IV). Field emission scanning electron microscopy (FE-SEM; FEI Nova NanoSEM 450) was used to examine the morphologies of the catalyst surface. Moreover, Fourier-transform infrared spectroscopy (FTIR) (Thermo, AVATAR) using a pellet generated by integrating the powder sample with KBr was used to observe functional groups of MgO nanoparticles.

Results and discussion
The characterization of the catalyst. Figure 2 presents the morphological properties of the nanoparticles as determined by the scanning electron microscopy (SEM) analysis, performed before starting the reaction. As can be seen, it is obvious that the magnesium oxide nanoparticles had a porous, spongy structure. Figure 3 illustrates, in the amorphous, shape, two peaks seen in 2θ = 43 and 62, illustrating the presence of cubic MgO, and the peaks can be assigned to a pure phase of MgO. The FTIR spectrum was also tested for investigation and identification of the catalyst surface's functional groups. It is well known that MgO chemisorbs H 2 O and CO 2 molecules from the atmosphere due to its surface acid-base properties 20 . The major peaks appearing in the FT-IR spectra may be assigned to the following modes: (i) a broad vibration band around 2800-3700 cm −1 can be associated to OH stretching vibrations of the surface-bonded (or) adsorbed water, which was introduced in precursor solution (ii) the peak around 1629 cm −1 is devoted to OH bending vibrations of water molecules. According to Fig. 4, a strong peak band was detected at a wavelength of 528 cm −1 , due to asymmetric vibrations of the Mg-O band. Peaks observed at 850 cm −1 corresponded to C=O stretching vibrations. (Comparative Study of Microwave and Conventional Methods for the Preparation and Optical Properties of Novel MgO-Micro and Nano-Structures) (MgO Nanoparticles Prepared By Microwave-Irradiation Technique and Its Seed Germination Application). The surface hydroxyl groups have been recognized to play an important role in the photocatalytic reaction since they can inhibit the recombination of photogenerated charge carriers, and also interact with the photogenerated holes to produce active oxygen species 20 .   www.nature.com/scientificreports/ Impact of initial pH. The literature review revealed that AOPs are completely pH-dependent 21 . Hence, in this study, at the fixed hydrogen peroxide content of 1 mM, the pH values were changed from 3 to 11 to investigate the changes in the removal efficiency. The maximum removal efficiency (73%) by the UV/H 2 O 2 /MgO method was attained at a pH of 3 (Fig. 5). These findings are attributed to the surface properties of the adsorbent and the ionization/degradation of the adsorbate. The number of hydrogen ion increases gradually with decreasing pH. When H + is adsorbed, the positive charge on the nanoparticle's surface increases and, in turn, the electrostatic force between the cationic charge on the surface of the nanoparticle and the negative DEX molecule enhances, increasing the adsorption rate. It was found that the performance declined sharply when pH was raised. For example, a 45% decrease was seen in removal efficiency at a pH value of 11 within 30 min. As can be seen, the degradation rate remained unchanged after 20 min and was insignificant after 30 min. Therefore, reaction times between 0 and 30 min were selected for the rest of the experiments. Furthermore, a decrease in the removal efficiency of the UV/H 2 O 2 in alkaline conditions can be caused by a reaction between H 2 O 2 and solution alkalinity; this causes hydroxyl radicals to go down. Moreover, in comparison with a neutral pH, the nanoparticles are accumulated in acidic conditions; as a result, the catalyst's effective surface area was enhanced 22 .
Impact of H 2 O 2 dosage. In this study, under the following conditions: pH 3, DEX content of 20 mg/L, and catalyst dosage of 0.05 g/L, different initial contents of hydrogen peroxide (1-8 mM) were tested. According to the results presented in Fig. 6, the removal efficiency increased to 87% when the concentration of H 2 O 2 was raised to 5 mM. It should be noted that, when the H 2 O 2 concentration exceeded 5 mM, the removal efficiency started to decline. An excessive increase in H 2 O 2 concentration causes part of ·OH to be inhibited and then HO 2 is produced, which has a lower oxidation potential than ·OH (Eq. (1)) 23 . Also, this decrease in performance can be because of the continuous degradation of H 2 O 2 into oxygen and water as shown in Eq. (1) 24 .
(1)   www.nature.com/scientificreports/ Impact of initial DEX concentration. In photocatalytic processes, how the initial concentration of the pollutant affects the removal efficiency is of great importance. Figure 7 shows the impact of the initial DEX content on the removal efficiency in UV/H 2 O 2 /MgO. As can be seen, with an increased DEX concentration from 5 to 30 mg/L, the removal efficiency declined. And, 65% of DEX was degraded at a concentration of 30 mg/L. Within 5 min of the reaction and an initial DEX content of 5 mg/L, a 90% removal efficiency was reached (Fig. 7). The decrease in the removal rate by increasing the concentration of DEX can be attributed to the fact that at all concentrations, the amount of nanoparticles, contact time, and pH are the same. As a result, the amount of radicals produced is similar at all concentrations. Naturally, it is expected to see lower DEX degradation at lower concentrations. By contrast, at a lower initial concentration, the number of active sites on the catalyst's surface capable of degrading DEX increases. Furthermore, ultraviolet light cannot penetrate effectively into the solution when there are higher concentrations of DEX 25 .
Impact of the dose of MgO. In Fig. 8, it is shown how the changes in magnesium oxide (0.01-0.2 g/L) affected the removal efficiency of the pollutant in photo-oxidation. As can be seen, the removal efficiency went up with the increase in the dose of MgO. Nevertheless, when the dosage exceeded 0.05 g/L, the removal rate declined. At higher doses, there are more active sites and free electrons in the conductor, resulting in the generation of more hydroxyl radicals that can take part in the degradation 26   ), hole (h + ), and hydroxyl radical (·OH) scavengers, respectively 28 . The results show three types of inhibition, corresponding to the three active species in the UV/H 2 O 2 /MgO process. From Fig. 9 it can be see that 87% of DEX can be removed in 30 min without a scavenger (Control). However, with the addition of AA, EDTA, and TBA, DEX removal efficiency decreased to 73.5%, 64.6% and 34.8%, respectively (Fig. 9). The rate of DEX degradation during the reaction process was less affected by the addition of AA (a scavenger of O 2 ·− ). Since TBA is a known ·OH scavenger 29  www.nature.com/scientificreports/ the presence of TBA clearly shows that the reaction with ·OH was the predominant active specie contributing to DEX removal. Furthermore, the decrease in DEX degradation in the presence of EDTA as a hole scavenger confirms h + photogeneration. The hole reactive species directly or indirectly oxidizes DEX compounds by generating hydroxyl radicals through the oxidation of water molecules. Therefore, the main mechanism was discovered to be in the form of ·OH-driven reactions, confirming those ·OH radicals were key species in the UV/H 2 O 2 /MgO process in DEX degradation, as described in the following Eqs. (2)-(6) 28,30 : This result corresponds with Akbari et al. 30 study that stated hydroxyl radicals are the main mechanism in ciprofloxacin antibiotic removal using S, N-doped MgO nanoparticles under UVA-LED. TOC analysis and mineralization. In this study, the content of TOC was determined because DEX is initially converted to other degradation byproducts that are still organic. Thus, we determined the mineralization of DEX by recording TOC concentrations during the process. The TOC and COD concentrations of the samples were determined under the selected conditions (Fig. 10). It was found that the initial TOC was determined at  www.nature.com/scientificreports/ 53.8 mg/L, and it declined to 23.5 mg/L after the exertion of the UV/H 2 O 2 /MgO process for 30 min, illustrating a mineralization rate of 56%. Accordingly, COD was reduced by up to 65%. However, at the same time of contact, the rate of DEX removal was 87%. Thus, it is claimed that for more mineralization, more contact time is required. For instance, the TOC removal rate increased to 98% within 120 min. It should be pointed out that lower by-products can be generated when a suitable contact time is regarded for reaching the mineralization rate of interest by the UV/H 2 O 2 /MgO process. It should be noted that, in the application of photocatalytic reactions, intermediates must be detected and eco-toxicological examinations should be performed.
Comparison of the processes. In this study, the UV/H 2 O 2 process was run in the presence and absence of the MgO catalyst. Also, the results of the UV and UV/MgO processes were compared. As indicated in Fig. 11, only 8% of the pollutant was degraded via the UV application within 30 min. Moreover, the performance of the UV/MgO process was nearly 17%, which may be because of the low adsorption rate that occurred on the surface of magnesium oxide. It should be noted that there was a dramatic difference between the removal efficiency rates of the UV/H 2 O 2 photo-oxidation and the UV/H 2 O 2 /MgO process, which were found to be 61% and 87%, respectively. The activity of magnesium oxide in catalyzing oxidation decay was relative to the surface acid-base properties of the oxide. Water molecules can be adsorbed on the magnesium oxide's surface due to the unsaturated state of surface electrons. As a result, surface hydroxyl groups may be formed. These groups play a basic role in the acid-base characterizations of magnesium oxide. Therefore, the process can be catalyzed well due to the surface hydroxyl groups. Thus, it is expected to see more DEX removal in the presence of magnesium oxide.
Investigation of process kinetics. The behavior of DEX removal was studied by both the linear forms of pseudo-first and second-order kinetic models 31 as expressed in Eqs. (7) and (8). www.nature.com/scientificreports/ Here, C 0 and C t show DEX concentration at times 0 and t (min), respectively. k 1 (min −1 ) and k 2 (mg/L.min) are assigned to the first and second-order kinetic constants, respectively. Figures 12 and 13 show pseudo-first and second-order kinetic models obtained by plotting Ln (c t /c 0 ) and 1/c t -1/c 0 against reaction time. The values of k 1 and k 2 obtained by the corresponding kinetic models are given in Degradation pathway of DEX. The hydroxyl radical interacts with organic pollutants quickly and strengthens C-unsaturated bonds. Because DEX molecules contain many OH groups, they are unstable to oxidation with a ·OH radical 32 . In the present study, the intermediates were determined using the LC-MS method to determine the degradation pathway of DEX in the UV/H 2 O 2 /MgO process. Based on degradation intermediates of DEX and previously published data 32,33 , two possible pathways of DEX degradation were proposed (Fig. 14).   , the products of (B), (C), and (D) could be attained by the cleavages of the bonds, which was related to the direct degradation of DEX compounds via the process of photodegradation. In the second pathway, the hydroxyl radical attack on the methylene group results in the mineralization of two carbon atoms and the creation of a ketone group on the remaining structure. The preferred loss and degradation in DEX are HF, which are continued by the combined losses of the HF and H 2 O molecules. (Intermediate D) 34 . Two water molecules are released after the ring breaks up. On further attack by the hydroxyl radical and after the breaking of the benzene rings, DEX was degraded to less refractory intermediate compounds, and thereby these compounds mineralized into CO 2 and H 2 O 18 .

Conclusion
In this study, dexamethasone was degraded by using H 2 O 2 and hydroxyl radical generation-based AOP processes. It was found that, with decreasing pH and initial DEX concentration, the removal efficiency of the UV/H 2 O 2 / MgO method improved. Also, the following values were determined to be the optimum conditions: hydroxyl and magnesium oxide nanoparticle concentrations up to 5 mM and 0.05 g/L, an initial concentration of 20 mg/L, and a contact time of 30 min. The kinetic response illustrated that the obtained data followed the pseudo-firstorder kinetic model. The findings also indicated that the UV/H 2 O 2 method could dramatically degrade the pollutant from an aqueous solution when the MgO catalyst was applied as a catalyst (mineralization rate of 98%). Further, the catalytic activity of magnesium oxide is attributed to the surface acid-base characterization of this oxide. Finally, the used process can be considered a suitable method for the removal of pharmaceuticals under optimum conditions.

Data availability
All data generated or analyzed during this study are included in this published article. The datasets generated and/ or analysed during the current study are available in the [Chemistry and Chemical biology] repository, [https:// www. sprin gerna ture. com/ gp/ autho rs/ resea rch-data-policy/ repos itori es-chem/ 12327 084]".